The 5 Biggest Lies About Pot—And How to Rebut Them

No, smoking marijuana doesn't lower your intelligence.

By Paul Armentano

1. Consuming marijuana lowers intelligence.

“The best evidence is that you lose, if you use marijuana as a teenager regularly, eight IQ points. I don’t know about the rest of the table, but I don’t have eight (IQ points) to lose.” —Washington Post columnist Ruth Marcus, July 27, 2014

The source of this oft-repeated claim is a 2012 longitudinal study by Madeline Meier and colleagues that associated the persistent use of cannabis prior to age 18 with lower IQ at mid-life. However, a separate review of Meier’s data, published in the same journal, disputed any direct link between cannabis use and declined IQ. That review argued that Meier’s team had failed to properly control for potential confounding factors, such as subjects’ socio-economic status. After accounting for these variables, the author theorized that the “true effect (on early onset cannabis use and IQ) could be zero.”

More recent longitudinal studies further dismiss the notion that cannabis exposure negatively impacts IQ. A 2016 British study published in the Journal of Psychopharmacology assessed IQ and educational performance among a cohort of 2,235 marijuana-using teens and never users.

Authors concluded, “[T]he notion that cannabis use itself is causally related to lower IQ and poorer educational performance was not supported in this large teenage sample.”

Most recently, researchers at the University of California, Los Angeles and the University of Minnesota evaluated whether marijuana use was associated with changes in intellectual performance in two longitudinal cohorts of adolescent twins. Participants were assessed for intelligence at ages 9 to 12, before marijuana involvement, and again at ages 17 to 20. Investigators found no dose-response relationship between cannabis use and IQ decline. They also found no significant differences in performance among marijuana using subjects when compared to their non-using twins, concluding: "In the largest longitudinal examination of marijuana use and IQ change, ... we find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline. ...

[T]he lack of a dose-response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana."

2. Legalizing marijuana spawned the opioid epidemic.

“I think that when you see something like the opioid addiction crisis blossoming in so many states around this country, the last thing we should be doing is encouraging people (by regulating the adult use of marijuana).” —White House Press Secretary Sean Spicer, Feb. 23, 2017

Recent claims by the new administration that the use of marijuana may be linked to the rising use and abuse of opioids is unsupported by the available evidence. In reality, numerous studies find just the opposite result.

Specifically, researchers have linked legal marijuana access to lower rates of opioid use, hospitalization, and mortality. For instance, a 2016 study by investigators at the University of Michigan reported that chronic pain patients reduced their opioid use by 64 percent when cannabis became available. Israeli researchers documented similar results in a cohort of patients with treatment-resistant pain, reporting a 44 percent reduction in participants’ opioid consumption following the introduction of medical cannabis. Such a substitution effect can result in saved lives.

Writing last month in the journal Drug and Alcohol Dependence, authors reported that medical marijuana legalization laws are associated with significant reductions in hospitalizations due to opioid-related abuse or overdoses. Similarly, a 2014 study published in the journal JAMA Internal Medicine determined that legalizing medical cannabis is associated with as much as a 33 percent reduction in deaths attributable to the use of prescription opiates and heroin.

3. Cannabis smoke exposure is more damaging to the lungs than tobacco smoke.

“Smoking marijuana clearly damages the human lung.” —website of the American Lung Association, Marijuana and Lung Health

While some studies have linked chronic marijuana smoke exposure to higher instances of cough, phlegm, and bronchitis, science has refuted claims that cannabis inhalation causes the sort of serious adverse respiratory events commonly associated with smoking tobacco.

Specifically, the largest case-controlled study ever to investigate the respiratory effects of marijuana smoking reported that cannabis use was not associated with lung-related cancers, even among subjects who reported smoking more than 22,000 joints over their lifetime. "We hypothesized that there would be a positive association between marijuana use and lung cancer, and that the association would be more positive with heavier use," the study's lead researcher explained to the Washington Post. “What we found instead was no association at all, and even a suggestion of some protective effect" among marijuana smokers who had lower incidences of cancer compared to non-users.”

Reviews of similar studies from around the world have reached similar conclusions, finding “little or no association between the intensity, duration, cumulative consumption or age of start of cannabis smoke and the risk of lung cancer in all subjects or never smokers.”

Furthermore, unlike tobacco smoke, marijuana inhalation is not adversely associated with measurements of pulmonary function, such as forced expiratory volume and forced vital capacity even among long-term consumers.

Finally, cannabis consumers can mitigate their exposure to combustive gasses by utilizing a vaporizer, which heats marijuana flowers to a point where cannabinoid vapors form, but below the point of combustion. Clinical studies assessing vaporization as a mode of cannabis delivery report that these devices all but eliminate subjects’ potential exposure to gaseous toxins and are “an effective and apparently safe vehicle for THC delivery.”

4. States that have regulated the marijuana market have experienced a surge in violence.

"We're seeing real violence around that (marijuana legalization). Experts are telling me there's more violence around marijuana than one would think and there's big money involved." —U.S. Attorney General Jeff Sessions, Feb. 27, 2017

Contrary to the Attorney General’s claims, states that license the production and distribution of marijuana have not seen an uptick in violent crime. In fact, many jurisdictions have experienced a drop in violent crime post-legalization.

For example, a 2014 study published by researchers at the University of Texas reported that the enactment of “medical marijuana laws precedes a reduction in homicide and assault. ... In sum, these findings run counter to arguments suggesting the legalization of marijuana for medical purposes poses a danger to public health in terms of exposure to violent crime and property crimes.”

A federally funded study published by UCLA researchers also reported that the proliferation of medical cannabis retailers in urban areas “was not associated with violent crime or property crime rates,” and further speculated that the facilities may potentially reduce neighborhood crime since many retailers hire their own door security, utilize security cameras, and take other steps to deter would-be criminals.

Specific data from states that regulate recreational marijuana sales yield similar results. In Washington, where voters legalized adult use in 2012, violent crime fell ten percent statewide.

In Colorado, rates of violent crime and property crime fell in the city of Denver following legalization. Crime rates have similarly declined in Portland, Oregon according to a recent CATO think-tank policy report. Overall, CATO’s researchers concluded that concerns regarding the potential adverse effects of legalization laws on crime have largely been blown out of proportion.

“The absence of significant adverse consequences is especially striking given the sometimes dire predictions made by legalization opponents,” they reported.

5. Marijuana legalization is linked with a rise in traffic fatalities.

“Research proves ‘the terrible carnage out there on the roads caused by marijuana.’” —Robert Dupont, March 18, 2012

While some studies find a slightly elevated risk of motor vehicle accident in THC-positive drivers compared to drug free drivers, others do not. However, this elevated risk is well below the risk of accident associated with many other non-criminal behaviors, such as driving with two or more passengers. Further, this risk is significantly lower than the risk of accident associated with driving after consuming alcohol within legal limits, which may increase crash risk as much as 400 percent.

According to just published case-control data in the journal Injury Epidemiology, drivers testing positive for alcohol possessed an elevate risk of accident that was more than ten times higher (OR=16.33) than those of drivers who tested positive for THC (OR=1.54).

Most importantly, data from states that have liberalized marijuana’s legal status show no uptick in motor vehicle crashes. Writing in December in the American Journal of Public Health, investigators at Columbia University reported, "[O]n average, medical marijuana law states had lower traffic fatality rates than non-MML states. .... Medical marijuana laws are associated with reductions in traffic fatalities, particularly pronounced among those aged 25 to 44 years. ... It is possible that this is related to lower alcohol-impaired driving behavior in MML-states.” An assessment of traffic fatality data from Colorado yielded a similar conclusion, reporting, “[L]egalization is associated with a nearly 9 percent decrease in traffic fatalities, most likely to due to its impact on alcohol consumption.”

A recent Congressional Research Service report concluded, “[T]here was no trend identified in the percentage of drivers testing positive for marijuana (either marijuana only or marijuana in combination with other drugs/alcohol) for those involved in traffic fatalities and who were tested for drugs or alcohol” in Washington state post-legalization. A similar review of motor vehicle crash data in Oregon similarly reported regulating the adult marijuana market has not led to an increase in fatal accidents.